45, CORNHILL, E.C., AND 122, REGENT STREET, W., LONDON. 



23 



height above the level of the sea, or the difference of level between places of 

 comparison." 



Aneroid barometers, if occasionally compared with a mercurial standard, 

 are similar in their indications, and valuable ; but it must be remembered that 

 for exact scientific observation, the Aneroid barometer cannot be put into 

 comparison with the mercurial column for strict accuracy, although its con- 

 venient size and great sensibility render it most useful for obtaining observations 

 where a mercurial instrument is inconvenient to carry. 



Col. Sir H. James, R.E., in his Instructions for taking Meteorological Obser- 

 vations, says of the Aneroid : " This is a most valuable instrument, it is ex- 

 tremely portable. I have had one in use for upwards of ten years." 



One of the objects of Mr. Glaisher's experiments in balloons was "to 

 compare the readings of an Aneroid barometer with those of a mercurial baro- 

 meter." In the comparisons the readings of the mercurial barometer were 

 corrected for index-error and temperature. Speaking of Aneroid indications,* 

 Mr. Glaisher remarks : 



" A third (Aneroid) graduated down to five inches, and most carefully made 



and tested under the air- 

 pump before use, read the 

 same as the Mercurial 

 Barometer throughout the 

 high ascent to seven miles, 

 September 5th, 1862.f I 

 have taken this instrument 

 up with me in every sub- 

 sequent high ascent, and 

 it has always read the same 

 as the Mercurial Baro- 

 meter. These experiments 

 prove that an Aneroid can 

 be made to read correctly 

 at low pressures. 



" I may mention that on 

 several occasions, Aneroid 

 Barometers have been 

 taken whose graduations 

 have been too limited for 

 the heights reached : these 

 -p 24 have not broken or become 



* Travels in the Air. By F. Glaisher. Page 89. The Aneroid Barometer, 

 f Wolverhampton to Cold Weston, near Ludlow, September 5th, 18G2. 



